This invention relates to a continuous process for reducing the sulfur content of coal.
It is recognized that an air pollution problem exists whenever sulfur-containing fuels are burned. The resulting sulfur oxides are particularly objectionable pollutants because they can combine with moisture to form corrosive acidic compositions which can be harmful and/or toxic to living organisms in very low concentrations.
Coal is an important fuel and large amounts are burned in thermal generating plants primarily for conversion into electrical energy. Many coals generate significant and unacceptable amounts of sulfur oxides on burning. The extent of the air pollution problem arising therefrom is readily appreciated when it is recognized that coal combustion currently accounts for 60 to 65% of the total sulfur oxides emissions in the United States.
The sulfur content of coal, nearly all of which is emitted as sulfur oxides during combustion, is present in both inorganic and organic forms. The inorganic sulfur compounds are mainly iron pyrites, with lesser amounts of other metal pyrites and metal sulfates. The organic sulfur may be in the form of thiols, disulfides, sulfides and/or thiophenes chemically associated with the coal structure itself. Depending on the particular coal, the sulfur content may be primarily either inorganic or organic. Distribution between the two forms varies widely among various coals. For example, both Appalachian and Eastern interior coals are known to be rich in both pyritic and organic sulfur. Generally, the pyritic sulfur represents from about 25% to 70% of the total sulfur content in these coals.
Heretofore, it has been recognized to be highly desirable to reduce the sulfur content of coal prior to combustion. In this regard, a number of processes have been suggested for physically reducing the inorganic portion of the sulfur in coal. Organic sulfur cannot be physically removed from coal.
As an example, it is known that at least some pyritic sulfur can be physically removed from coal by grinding and subjecting the ground coal to froth flotation or washing processes. These processes are not fully satisfactory because a significant portion of the pyritic sulfur and ash are not removed. Attempts to increase the portion of pyritic sulfur removed have not been successful because these processes are not sufficiently selective. Because the processes are not sufficiently selective, attempts to increase pyrite removal can result in a large portion of coal being discarded along with ash and pyrite.
There have also been suggestions heretofore to remove pyritic sulfur from coal by chemical means. For example, U.S. Pat. No. 3,768,988 discloses a process for reducing the pyritic sulfur content of coal by exposing coal particles to a solution of ferric chloride. The patent suggests that in this process ferric chloride reacts with pyritic sulfur to provide free sulfur according to the following reaction process: EQU 2FeCl.sub.3 +FeS.sub.2 .fwdarw.3FeCl.sub.2 +2S
While this process is of interest for removing pyritic sulfur, a disadvantage of the process is that the liberated sulfur solids must then be separated from the coal solids. Processes involving froth flotation, vaporization and solvent extraction are proposed to separate the sulfur solids. All of these proposals, however, inherently introduce a second discrete process step, with its attendant problems and cost, to remove the sulfur from coal. In addition, this process is notably deficient in that it does not remove organic sulfur from coal.
In another approach, U.S. Pat. No. 3,824,084 discloses a process involving grinding coal containing pyritic sulfur in the presence of water to form a slurry, and then heating the slurry under pressure in the presence of oxygen. The patent discloses that under these conditions the pyritic sulfur (for example, FeS.sub.2) can react to form ferrous sulfate and sulfuric acid which can further react to form ferric sulfate. The patent discloses that typical reaction equations for the process at the conditions specified are as follows: EQU FeS.sub.2 +H.sub.2 O+7/2 O.sub.2 .fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4 EQU 2FeSO.sub.4 +H.sub.2 SO.sub.4 +1/2 O.sub.2 .fwdarw.Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O.
These reaction equations indicate that in this particular process the pyritic sulfur content continues to be associated with the iron as sulfate. While it apparently does not always occur, a disadvantage of this is that insoluble material, basic ferric sulfate, can be formed. When this occurs, a discrete separation procedure must be employed to remove this solid material from the coal solids to adequately reduce sulfur content. In addition elemental sulfur can be formed and deposit on the coal. Removing this elemental sulfur presents a problem. Several other factors detract from the desirability of this process. The oxidation of sulfur in the process does not proceed at a rapid rate, thereby limiting output for a given processing capacity. In addition, the above oxidation process is not highly selective in that considerable amounts of coal itself are oxidized. This is undesirable, of course, since the amount and/or heating value of the coal recovered from the process is decreased.
Heretofore, it has been known that coal particles could be agglomerated with hydrocarbon oils. For example, U.S. Pat. Nos. 3,856,668 and 3,665,066 disclose processes for recovering coal fines by agglomerating the fine coal particles with oil. U.S. Pat. Nos. 3,268,071 and 4,033,729 disclose processes involving agglomerating coal particles with oil in order to provide a separation of coal from ash. While these processes can provide some benefication of coal, better removal of ash and iron pyrite mineral matter would be desirable.
The discovery that coal particles, when agglomerated with oil, can be more amenable to sulfur removal in processes wherein the sulfur is oxidized to remove the sulfur from coal was first disclosed in U.S. patent application Ser. No. 916,666, filed June 19, 1978, commonly assigned. This latter discovery relates to an oxidation process, operated under elevated temperature and pressure.
In the oxidative contacting of an aqueous slurry of coal-oil particles with a gaseous oxygen-containing phase, such as air, an upflow reactor system is generally to be preferred. However, the coal-oil particles, usually associated with trapped air bubbles, tend to float on the aqueous phase. This property of floatability causes the coal-oil particles to move rapidly upward in the reactor system so that the residence time for the coal-oil particles in the reactor system is greatly reduced relative to that for the associated aqueous phase. Conventional baffling of the reactor system is generally without material effect on the disparity in residence times of the phases. Even though the shorter residence time of the coal-oil particles does lead to some oxidation, an effective means for increasing the residence time of the coal-oil particles would greatly improve the coal desulfurization without the necessity of resorting to a highly involved and less attractive reactor system.